An acceleration illusion caused by underestimation of stimulus velocity during pursuit eye movements: Aubert-Fleischl revisited

When the eyes pursue a fixation point that sweeps across a moving background pattern, and the fixation point is suddenly made to stop, the ongoing motion of the background pattern seems to accelerate to a higher velocity. Experiment I showed that this acceleration illusion is not caused by the sudden change in (i) the relative velocity between background and fixation point, (ii) the velocity of the retinal image of the background pattern, or (iii) the motion of the retinal image of the rims of the CRT screen on which the experiment was carried out. In experiment II the magnitude of the illusion was quantified. It is strongest when background and eyes move in the same direction. When they move in opposite directions it becomes less pronounced (and may disappear) with higher background velocities. The findings are explained in terms of a model proposed by the first author, in which the perception of object motion and velocity derives from the interaction between retinal slip velocity information and the brain's 'estimate' of eye velocity in space. They illustrate that the classic Aubert-Fleischl phenomenon (a stimulus seems to be moving slower when pursued with the eyes than when moving in front of stationary eyes) is a special case of a more general phenomenon: whenever we make a pursuit eye movement we underestimate the velocity of all stimuli in our visual field which happen to move in the same direction as our eyes, or which move slowly in the direction opposite to our eyes.     

Pursuit compensation during self-motion

The pattern of motion in the retinal image during self-motion contains information about the person's movement. Pursuit eye movements perturb the pattern of retinal-image motion, complicating the problem of self-motion perception. A question of considerable current interest is the relative importance of retinal and extra-retinal signals in compensating for these effects of pursuit on the retinal image. We addressed this question by examining the effect of prior motion stimuli on self-motion judgments during pursuit. Observers viewed 300 ms random-dot displays simulating forward self-motion during pursuit to the right or to the left; at the end of each display a probe appeared and observers judged whether they would pass left or right of it. The display was preceded by a 300 ms dot pattern that was either stationary or moved in the same direction as, or opposite to, the eye movement. This prior motion stimulus had a large effect on self-motion judgments when the simulated scene was a frontoparallel wall (experiment 1), but not when it was a three-dimensional (3-D) scene (experiment 2). Corresponding simulated-pursuit conditions controlled for purely retinal motion aftereffects, implying that the effect in experiment 1 is mediated by an interaction between retinal and extra-retinal signals. In experiment 3, we examined self-motion judgments with respect to a 3-D scene with mixtures of real and simulated pursuit. When real and simulated pursuits were in opposite directions, performance was determined by the total amount of pursuit-related retinal motion, consistent with an extra-retinal 'trigger' signal that facilitates the action of a retinally based pursuit-compensation mechanism. However, results of experiment 1 without a prior motion stimulus imply that extra-retinal signals are more informative when retinal information is lacking. We conclude that the relative importance of retinal and extra-retinal signals for pursuit compensation varies with the informativeness of the retinal motion pattern, at least for short durations. Our results provide partial explanations for a number of findings in the literature on perception of self-motion and motion in the frontal plane.     

Visual perception of direction when voluntary saccades occur. I. Relation of visual direction of a fixation target extinguished before a saccade to a flash presented during the saccade

In experiments designed to clarify the mechanisms underlying the normal stability of visual direction for stationary objects when voluntary saccades occur, Ss reported on the horizontal visual direction of a brief test [lash presented when the eye was at a specific point in the saccade (the trigger point) relative to a fixation target viewed and extinguished prior to the saccade. From these reports, PSEs (points of subjective equality) were calculated for the fixation target as measured by the test [lashes. The distance of the trigger point from the previous fixation position was systematically varied in each experiment. Different experiments required saccades of different lengths and directions. With the exception of the presentation of the test [lash the saccades were carried out in complete darkness so that the possible utilization of an extraretinal signal regarding the eye movement (change in eye position, the intention to turn the eye, or a change of attention related to the eye movement) in the determination of visual direction could be observed uncomplicated by a continuing visual context. According to classical theories, an extraretinal signal proportional to the change in eye position acts to maintain direction constancy by compensating for the Shift of the retinal image resulting from the movement of the eye. In general, direction constancy was not preserved in the present experiments, and thus the data would not be predicted by classical theories. However, the PSE varied with distance of the trigger point from the fixation target. Since this displacement of PSE from the trigger point was in the correct direction for compensation, the presence of an extraretinal signal was confirmed. However, the growth of this signal appears to be time-locked to the saccade rather than locked to eye position; it is suggested that this growth takes place over a time period which is longer than the duration of the saccade itself.     

Rapid adaptation of the vestibulo-ocular reflex and induced self-motion perception

When a rigid object moves toward the eye, it is usually perceived as being rigid. However, in the case of motion away from the eye, the motion and structure of the object are perceived nonveridically, with the percept tending to reflect the nonrigid transformations that are present in the retinal image. This difference in response to motion to and from the observer was quantified in an experiment using wire-frame computer-generated boxes which moved toward and away from the eye. Two theoretical systems are developed by which uniform three-dimensional velocity can be recovered from an expansion pattern of nonuniform velocity vectors. It is proposed that the human visual system uses two similar systems for processing motion in depth. The mechanism used for motion away from the eye produces perceptual errors because it is not suited to objects with a depth component.     

Locus of habituation in the human newborn

There is some controversy concerning the youngest age at which an infant will habituate to a visual stimulus or will prefer a novel to a familiar pattern. One suggestion has been that apparently successful reports of habituation and dishabituation in the newborn baby are attributable to retinal adaptation. This interpretation was tested in two experiments. In both experiments monocular conditions of viewing were used: newborns were habituated with one eye as the 'seeing' eye, and posthabituation novelty preferences investigated with the other eye. Significant preferences were found both for a novel colour (experiment 1) and for a novel shape (experiment 2), which implies that a retinal-adaptation model can be ruled out. It is suggested that the habituation effects and the subsequent novelty preferences found in the experiments are most reasonably interpreted as a function of memory formation, and evidence is presented for the storage of visual experience from birth. The results also demonstrate some form of binocular interaction in the newborn.     

Aftereffects in binocular rivalry

Five experiments are reported in which the aftereffect paradigm was applied to binocular rivalry. In the first three experiments rivalry was between a vertical grating presented to the left eye and a horizontal grating presented to the right eye. In the fourth experiment the rivalry stimuli consisted of a rotating sectored disc presented to the left eye and a static concentric circular pattern presented to the right. In experiment 5 rivalry was between static radiating and circular patterns. The predominance durations were systematically influenced by direct (same eye) and indirect (interocular) adaptation in a manner similar to that seen for spatial aftereffects. Binocular adaptation produced an aftereffect that was significantly smaller than the direct aftereffect, but not significantly different from the indirect one. A model is developed to account for the results; it involves two levels of binocular interaction in addition to monocular channels. It is suggested that the site of spatial aftereffects is the same as that for binocular rivalry, rather than sequentially prior.     

The interaction of perceived distance with the perceived direction of visual motion during movements of the eyes and the head.

A horizontally moving target was followed by rotation of the eyes alone or by a lateral movement of the head. These movements resulted in the retinal displacement of a vertically moving target from its perceived path, the amplitude of which was determined by the phase and amplitude of the object motion and of the eye or head movements. In two experiments, we tested the prediction from our model of spatial motion (Swanston, Wade, & Day, 1987) that perceived distance interacts with compensation for head movements, but not with compensation for eye movements with respect to a stationary head. In both experiments, when the vertically moving target was seen at a distance different from its physical distance, its perceived path was displaced relative to that seen when there was no error in perceived distance, or when it was pursued by eye movements alone. In a third experiment, simultaneous measurements of eye and head position during lateral head movements showed that errors in fixation were not sufficient to require modification of the retinal paths determined by the geometry of the observation conditions in Experiments 1 and 2.     

The interaction of perceived distance with the perceived direction of visual motion during movements of the eyes and the head

A horizontally moving target was followed by rotation of the eyes alone or by a lateral movement of the head. These movements resulted in the retinal displacement of a vertically moving target from its perceived path, the amplitude of which was determined by the phase and amplitude of the object motion and of the eye or head movements. In two experiments, we tested the prediction from our model of spatial motion (Swanston, Wade, & Day, 1987) that perceived distance interacts with compensation for head movements, but not with compensation-for eye movements with respect to a stationary head. In both experiments, when the vertically moving target was seen at a distance different from its physical distance, its perceived path was displaced relative to that seen when there was no error in pereived distance, or when it was pursued by eye movements alone. In a third experiment, simultaneous measurements of eye and head position during lateral head movements showed that errors in fixation were not sufficient to require modification of the retinal paths determined by the geometry of the observation conditions in Experiments 1 and 2.     

Retinal and extraretinal information in movement perception: how to invert the Filehne illusion

During a pursuit eye movement made in darkness across a small stationary stimulus, the stimulus is perceived as moving in the opposite direction to the eyes. This so-called Filehne illusion is usually explained by assuming that during pursuit eye movements the extraretinal signal (which informs the visual system about eye velocity so that retinal image motion can be interpreted) falls short. A study is reported in which the concept of an extraretinal signal is replaced by the concept of a reference signal, which serves to inform the visual system about the velocity of the retinae in space. Reference signals are evoked in response to eye movements, but also in response to any stimulation that may yield a sensation of self-motion, because during self-motion the retinae also move in space. Optokinetic stimulation should therefore affect reference signal size. To test this prediction the Filehne illusion was investigated with stimuli of different optokinetic potentials. As predicted, with briefly presented stimuli (no optokinetic potential) the usual illusion always occurred. With longer stimulus presentation times the magnitude of the illusion was reduced when the spatial frequency of the stimulus was reduced (increased optokinetic potential). At very low spatial frequencies (strongest optokinetic potential) the illusion was inverted. The significance of the conclusion, that reference signal size increases with increasing optokinetic stimulus potential, is discussed. It appears to explain many visual illusions, such as the movement aftereffect and center-surround induced motion, and it may bridge the gap between direct Gibsonian and indirect inferential theories of motion perception.     

Observations on the adaptation of induced motion

Induced motion (IM) was measured before and after a 10-min adaptation period during which subjects viewed the IM display without judging IM magnitude. The inducing stimulus was a rectangle, which contains both horizontal and vertical reference detail. The magnitude of IM was significantly lower following the adaptation period. This result is inconsistent with the hypothesis that adaptation of IM represents an instance of perceptual learning wherein the contribution of relative motion to motion perception is reduced. In a separate study, similar results were obtained when the inducing stimulus was a single vertical bar presented either to the left or to the right of the fixation stimulus. In addition, adaptation was obtained when the location of the inducing bar was changed during test measures, demonstrating that this effect is not specific to the retinal locus of the adaptation stimulus.     

Measuring the size of mental images

Mental images seem to have a size; the experimental problem was to map that image size onto a scale of physical measurement. To this end, two experiments were conducted to measure the size of mental images in degrees of visual angle. In Experiment 1, college students employed light pointers to indicate the horizontal extent of projected mental images of words (the letter string, not the referent). Imagined words covered about 1.0 degress of visual angle per letter. In Experiment 2, a more objective eye-movement response was used to measure the visual angle size of imagined letter strings. Visual angle of eye movement was found to increase regularly as the letter distance between the fixation point and a probed letter position increased. Each letter occupied about 2.5 degrees of visual angle for the four-letter strings in the control/default size condition. Possible relations between eye movements and images are discussed.     

Perception of self-motion from visual flow

Accurate and efficient control of self-motion is an important requirement for our daily behavior. Visual feedback about self-motion is provided by optic flow. Optic flow can be used to estimate the direction of self-motion (‘heading’) rapidly and efficiently. Analysis of oculomotor behavior reveals that eye movements usually accompany self-motion. Such eye movements introduce additional retinal image motion so that the flow pattern on the retina usually consists of a combination of self-movement and eye movement components. The question of whether this ‘retinal flow’ alone allows the brain to estimate heading, or whether an additional ‘extraretinal’ eye movement signal is needed, has been controversial. This article reviews recent studies that suggest that heading can be estimated visually but extraretinal signals are used to disambiguate problematic situations. The dorsal stream of primate cortex contains motion processing areas that are selective for optic flow and self-motion. Models that link the properties of neurons in these areas to the properties of heading perception suggest possible underlying mechanisms of the visual perception of self-motion.     

The integration of orientation information in the motion correspondence problem

We examine how differently oriented components contribute to the discrimination of motion direction along a horizontal axis. Stimuli were two-frame random-dot kinematograms that were narrowband filtered in spatial frequency. On each trial, subjects had to state whether motion was to the left or the right. For each stimulus condition, Dmax (the largest displacement supporting 80% correct direction discrimination performance) was measured. In experiment 1, Dmax was measured for orientationally narrowband stimuli as a function of their mean orientation. Dmax was found to increase as the orientation of the stimuli became closer to the axis of motion. Experiment 2 used isotropic stimuli in which some orientation bands contained a coherent motion signal, and some contained only noise. When the noise band started at vertical orientations and increased until only horizontal orientations contained a coherent motion signal, Dmax increased slightly. This suggests that near-vertical orientations interfere with motion perception at large displacements when they contain a coherent motion signal. When the noise band started at horizontal and increased until only vertical orientations contained the motion signal, Dmax decreased steadily. This implies that Dmax depends at least partly on the most horizontal motion signal in the stimulus. These results were contrasted with two models. In the first, the visual system utilises the most informative orientations (nearest horizontal). In the second, all available orientations are used equally. Results supported an intermediate interpretation, in which all orientations are used but more informative ones are weighted more heavily.     

Perceived localizations and eye movements with action‐generated and computer‐generated vanishing points of moving stimuli

When observers localize the vanishing point of a moving target, localizations are reliably displaced beyond the final position, in the direction the stimulus was travelling just prior to its offset. We examined modulations of this phenomenon through eye movements and action control over the vanishing point. In Experiment 1 with pursuit eye movements, localization errors were in movement direction, but less pronounced when the vanishing point was self‐determined by a key press of the observer. In contrast, in Experiment 2 with fixation instruction, localization errors were opposite movement direction and independent from action control. This pattern of results points at the role of eye movements, which were gathered in Experiment 3. That experiment showed that the eyes lagged behind the target at the point in time, when it vanished from the screen, but that the eyes continued to drift on the targets' virtual trajectory. It is suggested that the perceived target position resulted from the spatial lag of the eyes and of the persisting retinal image during the drift.     

Experiments on the locus of induced motion

Two experiments examined the locus of induced motion effects. The first used a subjective technique to test for the presence of retinal slippage due to systematic eye movements when an observer fixates a test spot in the center of a horizontally moving rectangle. The second experiment tested for “local” retinal effects by presenting test and inducing figures dichoptically. There was no evidence of retinal slippage under conditions where induced motion was not discriminable from real motion. Moreover, good induction was produced across eyes. Implications for the locus of induced motion effects are discussed.     

Additivity of retinal and pursuit velocity in the perceptions of depth and rigidity from object-produced motion parallax

Two psychophysical experiments were conducted to investigate the mechanism that generates stable depth structure from retinal motion combined with extraretinal signals from pursuit eye movements. Stimuli consisted of random dots that moved horizontally in one direction (ie stimuli had common motion on the retina), but at different speeds between adjacent rows. The stimuli were presented with different speeds of pursuit eye movements whose direction was opposite to that of the common retinal motion. Experiment 1 showed that the rows moving faster on the retina appeared closer when viewed without eye movements; however, they appeared farther when pursuit speed exceeded the speed of common retinal motion. The 'transition' speed of the pursuit eye movement was slightly, but consistently, larger than the speed of common retinal motion. Experiment 2 showed that parallax thresholds for perceiving relative motion between adjacent rows were minimum at the transition speed found in experiment 1. These results suggest that the visual system calculates head-centric velocity, by adding retinal velocity and pursuit velocity, to obtain a stable depth structure.     

Visually perceived eye level and horizontal midline of the body trunk influenced by optic flow

The eye level and the horizontal midline of the body trunk can serve, respectively, as references for judging the vertical and horizontal egocentric directions. We investigated whether the optic-flow pattern, which is the dynamic motion information generated when one moves in the visual world, can be used by the visual system to determine and calibrate these two references. Using a virtual-reality setup to generate the optic-flow pattern, we showed that judged elevation of the eye level and the azimuth of the horizontal midline of the body trunk are biased toward the positional placement of the focus of expansion (FOE) of the optic-flow pattern. Furthermore, for the vertical reference, prolonged viewing of an optic-flow pattern with lowered FOE not only causes a lowered judged eye level after removal of the optic-flow pattern, but also an overestimation of distance in the dark. This is equivalent to a reduction in the judged angular declination of the object after adaptation, indicating that the optic-flow information also plays a role in calibrating the extraretinal signals used to establish the vertical reference.     

Horizontal saccadic eye movements enhance the retrieval of landmark shape and location information

Recent work has demonstrated that horizontal saccadic eye movements enhance verbal episodic memory retrieval, particularly in strongly right-handed individuals. The present experiments test three primary assumptions derived from this research. First, horizontal eye movements should facilitate episodic memory for both verbal and non-verbal information. Second, the benefits of horizontal eye movements should only be seen when they immediately precede tasks that demand right and left-hemisphere processing towards successful performance. Third, the benefits of horizontal eye movements should be most pronounced in the strongly right-handed. Two experiments confirmed these hypotheses: horizontal eye movements increased recognition sensitivity and decreased response times during a spatial memory test relative to both vertical eye movements and fixation. These effects were only seen when horizontal eye movements preceded episodic memory retrieval, and not when they preceded encoding (Experiment 1). Further, when eye movements preceded retrieval, they were only beneficial with recognition tests demanding a high degree of right and left-hemisphere activity (Experiment 2). In both experiments the beneficial effects of horizontal eye movements were greatest for strongly right-handed individuals. These results support recent work suggesting increased interhemispheric brain activity induced by bilateral horizontal eye movements, and extend this literature to the encoding and retrieval of landmark shape and location information.     

Measurement of visual aftereffects and inferences about binocular mechanisms in human vision

There is conflicting evidence concerning the characteristics of binocular channels in the human visual system with respect to the existence of a 'pure' binocular channel that responds only to simultaneous stimulation of both eyes. Four experiments were conducted to resolve these discrepancies and to evaluate the evidence for the existence of such an exclusive binocular channel. In the first three studies, tilt aftereffects were measured after monocular adaptation. The relative sizes of the direct, interocularly transferred, and binocular aftereffects were not influenced by the configuration of the adapting pattern (experiment 1), or by the eye used for adaptation (experiment 2). There were also consistent interobserver differences in the relative sizes of the aftereffect seen after monocular adaptation (experiment 3). Taken together, these data raise questions about the appropriateness of a monocular adaptation paradigm for evaluating the presence of a pure binocular channel in observers with normal binocular vision. In experiment 4, in which the paradigm of alternating monocular adaptation was used, data were obtained that are consistent with the presence of a pure binocular channel.     

Changes in Saccadic and Manual Motor Control After Ocular Smooth Pursuit Adaptive Modifications

The gain of ocular smooth pursuit responses can be adaptively modified under certain circumstances. Evidence that these modifications are caused, at least partly, by changes at a sensory level comes from the fact that subsequent manual tracking movements generated during visual fixation are also modified in a similar manner. The question addressed in the present experiment was whether the modifications are constrained specifically to responses such as these, which are driven by retinal image motion, or whether they can also influence movements that make use of retinal image position. This was accomplished by comparing manual and oculomotor responses to step changes in target position before and after smooth pursuit adaptation. The results showed that, as with manual tracking movements, these responses were also modified by the adaptive procedure, although to a lesser extent. In particular, following a 20-min period in which subjects (N = 4) were submitted to a procedure designed to increase the gain of the smooth pursuit system, manual step tracking movements and ocular saccades displayed larger amplitudes than those shown prior to the adaptation. In addition, these gain changes were accompanied by appropriate alterations in response kinematics. Taken together, these results suggest that the mechanisms responsible for adaptive modifications in the smooth pursuit system are also able to more generally influence the processing of visuospatial information. The possible neurophysiological substrate underlying these mechanisms is discussed.